The vertebrate spine is the axis of the skeleton on which all of the body parts “hang”. In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar spine sits upon the sacrum, which then attaches to the pelvis, and in turn is supported by the hip and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which act as joints but allow known degrees of flexion, extension, lateral bending, and axial rotation.
The typical vertebra has a thick anterior bone mass called the vertebral body, with a neural (vertebral) arch that arises from the posterior surface of the vertebral body. The centra of adjacent vertebrae are supported by intervertebral discs. Each neural arch combines with the posterior surface of the vertebral body and encloses a vertebral foramen. The vertebral foramina of adjacent vertebrae are aligned to form a vertebral canal, through which the spinal sac, cord and nerve rootlets pass. The portion of the neural arch which extends posteriorly and acts to protect the spinal cord's posterior side is known as the lamina. Projecting from the posterior region of the neural arch is the spinous process.
The intervertebral disc primarily serves as a mechanical cushion permitting controlled motion between vertebral segments of the axial skeleton. The normal disc is a unique, mixed structure, comprised of three component tissues: the nucleus pulpous (“nucleus”), the annulus fibrosus (“anulus”) and two vertebral end plates. The two vertebral end plates are composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus act to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae.
The spinal disc and/or vertebral bodies may be displaced or damaged due to trauma, disease, degenerative defects, or wear over an extended period of time. One result of this displacement or damage to a spinal disc or vertebral body may be chronic back pain.
A disc herniation occurs when the anulus fibers are weakened or torn and the inner tissue of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal anulus confines. The mass of a herniated or “slipped” nucleus tissue can compress a spinal nerve, resulting in leg pain, loss of muscle control, or even paralysis. Alternatively, with discal degeneration, the nucleus loses its water binding ability and deflates, as though the air had been let out of a tire. Subsequently, the height of the nucleus decreases causing the anulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular tears may occur, which may contribute to persistent or disabling back pain. Adjacent, ancillary spinal facet joints will also be forced into an overriding position, which may create additional back pain.
Whenever the nucleus tissue is herniated or removed by surgery, the disc space will narrow and may lose much of its normal stability. In many cases, to alleviate back pain from degenerated or herniated discs, the disc is removed along with all or part of at least one neighboring vertebrae and is replaced by an implant that promotes fusion of the remaining bony anatomy. Some examples of such implants include those described in U.S. Pat. Nos. 6,520,993, 5,893,889, 5,683,465, 5,674,294, 5,458,643, 5,306,309, and 4,579,769.
While this treatment may help alleviate the pain once the vertebrae have been successfully fused together, there remains the possibility that the surgical procedure may not successfully or fully bring about the intended fusion. The success or failure of spinal fusion may depend upon several factors. For instance, the spacer—or implant or cage—used to fill the space left by the removed disc and bony anatomy must be sufficiently strong to support the spine under a wide range of loading conditions. The spacer should also be configured so that it is likely to remain in place once it has been positioned in the spine by the surgeon. Additionally, the material used for the spacer should be a biocompatible material and should have a configuration that promotes bony ingrowth.
As a result, the design of the implant should provide sufficient rigidity and strength to resist deformation when loading forces are applied to it. Likewise, the implant should sufficiently resist sliding or movement of the implant as a result of torsional or shearing loads. Often, these parameters lead designers to select predominantly solid structures made of bone or of radio opaque materials such as titanium.
For example, U.S. Pat. No. 6,547,823 describes an intervertebral implant made of cortical ring cut from a diaphysis of a long bone. The implant is generally solid with a through-hole extending from the top surface of the implant to the bottom surface.
U.S. Pat. No. 6,245,108 likewise illustrates how the design parameters described above often lead to a predominantly solid structure. The patent describes an interbody fusion implant having a top, bottom, front, back, and sides. A large through-hole is provided from the top surface to the bottom, while the front, back, and sides only have substantially smaller openings.
While it is generally known that providing a through-hole from the top to bottom surfaces helps promote bone growth in the treated area, the structure of the front, back, and side walls often substantially obscure or do not permit confirmation that fusion is taking place as desired. This difficulty of subsequently confirming that fusion is adequately taking place is particularly problematic when the spacer is made of radio opaque materials. Thus, while the use of radio opaque materials can be beneficial for providing greater structural rigidity of the spacer and for helping the physician to identify the position of the implant during insertion, these materials can have the disadvantage of limiting the ability to later confirm the implant has served its intended purpose of fusing the treated area.
Another disadvantage that results from prior art spacer designs is that of solid sidewalls or sidewalls with limited openings limits fusion of the treated area principally to the opening on the upper and lower sides of the spacer, if one is provided. Thus, while a spacer having an essentially closed or solid structure may maintain rigidity of the spacer, it does not provide optimal conditions for fusion to take place.
Instrumentation and specialized tools for insertion of an intervertebral implant is yet another design parameter to consider when designing a spacer. Spinal fusion procedures can present several challenges because of the small clearances around the spacer when it is being inserted into position. For instance, the instrumentation used may securely grip the implant on opposing sides or surfaces. In U.S. Pat. No. 6,520,933, for example, the superior and inferior surfaces have one or more regions in which no gripping teeth are present. These protrusion-free zones enable the implant to be grasped and manipulated by elongate rectangular blades. Notably, these protrusion-free zones are not formed as channels cut into the surface of the implant in order to maintain the strength and integrity of the implant so that it is less prone to failure. Thus, the clearance required in order to insert the spacer must be higher than the spacer itself in order to accommodate the instrumentation of the '933 patent. For this reason, distraction of the treated area typically is greater than the implant itself.
Similarly, when the gripping tools used to manipulate and insert the implant are on the sides of the spacer, additional clearance typically is needed in order to accommodate the added width of the insertion tool blades. Such increases in height or width of the profile of the spacer when coupled or in communication with instrumentation means that additional space is needed in order to insert the spacer. In some circumstances, providing for this additional clearance space can be difficult to achieve.
Thus, despite known devices that promote fusion of a treated area of the spine, there remains a need for spacer designs that optimize bony ingrowth, have structural rigidity to support the spine under a variety of loading conditions, allow for insertion through a lower profile, and provide for confirmation of the fusion process from a multiplicity of views.